This invention relates to a rotary drum device used in magnetic recording and playback apparatus such as VTRs, and particularly to the structure of a magnetic head and flying erasure head mounted on the rotary drum, and the method of mounting the heads on the rotary drum.
Many commercially available VTRs with edit functions incorporate a flying erasure head, which is a rotary erasing head operating in advance of the recording head to clean the seam section between an already recorded signal and a signal that is about to be recorded by erasing the old record, with the intention of ensuring a high picture quality while retaining the inherent edit function, as described in Japanese Utility Model Publication No. 62-7605, for example.
FIGS. 1A and 1B are diagrams showing the disposition of VTR heads including a flying ensure head 1 and the positional relation of the heads on recording tracks 5 and 5' based on the conventional VTR system. Because of analog recording adopted by many conventional VTR systems, recording (playback) heads 2 and 2' having different azimuth angles are disposed at positions of 180.degree. confrontation, as shown in FIG. 1A, so that a field of picture is formed by a revolution of one head on the rotary drum 4, i.e., a frame of picture is formed by two heads in one revolution of drum. In analog recording, the old record is erased for each frame of picture, instead of each field, and the flying erasure head 1 needs to erase two tracks in one revolution as shown in FIG. 1B by having a tracking width T.sub.W(FE) expressed as follows. EQU T.sub.W(FE) .perspectiveto.T.sub.W(Rec2) +T.sub.W(Rec2') ( 1)
where T.sub.W(Rec2) is the tracking width of recording (playback) head 2, and T.sub.W(Rec2') is the tracking width of recording (playback) head 2'.
Since the flying erasure head needs to erase two tracks of different azimuth angles simultaneously, it must have the azimuth angle set to the mean value of azimuth angles of the two recording tracks in consideration of the azimuth loss of the erasing signal. The flying erasure head 1 has the role of complete erasure of the old record, operating at an erasing frequency higher than the maximum recording frequency of the VTR system, with its wavelength being in the record demagnetization region, and therefore it has a gap length twice or more the gap length of the recording heads 2 and 2'.
The combination of the flying erasure head and recording heads based on the above-mentioned prior art is not intended for erasure of only one channel or a partial recording track on a track, which is required in digital-recording VTRs, for example. The higher erasing frequency of a large erasing current supplied to the flying erasure head will incur the head isolation problem when the recording frequency will be raised for increased recording density. Because of the completely different flying erasure head from the recording head in the conventional VTR system, they need different fabricating processes, resulting in a high manufacturing cost as the whole system.
Moreover, the long-gap head involves such problems as: (1) the gap section is prone to be decorticate and difficult to fabricate, (2) the gap edge section has an increased fringe effect to produce an increased actual erasing tracking width relative to the head tracking width. These problems jeopardize the adoption of long-gap head in a coming narrow-track VTR system.
A further problem of the conventional flying erasure head is crosstalk with another channel in such a system as a digital VTR, which deals with a recording frequency as high as several tens megahertz for extremely high transfer rate, where much higher erasing frequency is required. On this account, mounting a flying erasure head necessitates special head isolation measures at the stage of design.